Engines have used various forms of variable cam timing to improve engine operation over a variety of speed/load conditions. Variable cam timing adjusts the relative timing of the camshaft to the crankshaft, thus changing valve opening/closing timing relative to piston movement.
One type of variable cam timing system utilizes hydraulic oil pressure to adjust an actuator between the camshaft and crankshaft. For example, hydraulic vane type actuators may be used to move cam timing relative to crankshaft timing and piston position. The hydraulic actuator uses pressurized engine oil available during engine operation. However, when oil pressure is not available, a biasing spring and locking pin hold the actuator, and thus cam timing, in a default position, typically base timing which is in the earliest timing of the timing range.
However, the inventors herein have recognized several issues with such approaches. For example, there may be conflicting requirements in selecting the default (locking pin) position. Under some conditions (such as an engine shut-down and some starting/cranking conditions), it may be desirable to utilize retarded intake and exhaust valve timings relative to piston position. Such operation may be used to reduce flow of fresh air to the catalyst and to reduce cranking torque pulses that can increase vibration. However, under other conditions (such as some cold starting conditions and warmed-up idle conditions where oil pressure may be too low), it may be desirable to utilize a less retarded timing.
Thus, to address at least some of the above issues, a system for a vehicle with an engine having at least one cylinder with at least an intake valve and an exhaust valve may be provided. The system comprises a hydraulic variable valve actuator system configured to vary a timing of the intake valve in a first range and a timing of the exhaust valve in a second range, said actuator system configured to have a default intake valve timing to which intake valve timing is held when insufficient hydraulic pressure is available to move intake valve timing, and a default exhaust valve timing to which exhaust valve timing is held when insufficient hydraulic pressure is available to move exhaust valve timing, where said default intake valve timing is in a retarded portion of said first range and said default exhaust valve timing is in a retarded portion of said second range, and a hybrid propulsion system coupled to the engine.
In this way, by using default positions in which intake and exhaust valve timings are in retarded positions, it is possible to better utilize operation of the hybrid propulsion system. For example, improved engine shut-down and hot restart operation may be obtained to improve hybrid system operation. Further, it is possible to utilize the hybrid propulsion system to avoid engine operation where earlier timing may be desired, yet the actuator system is in it default position. For example, the hybrid system may be able to adjust engine speed to avoid or reduce conditions of insufficient hydraulic pressure. As another example, if earlier timing is desired during warmed-up idle conditions where desired speed is low (and thus pressure may be low), the engine may be shut-off.
As such, improved system performance may be achieved.